AZ L-85 Wastewater Treatment
Contractor Exam Questions And
Correct Answers (Verified Answers)
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1. Which process is primarily used to remove suspended solids from
wastewater?
A. Biological treatment
B. Sedimentation
C. Chlorination
D. Aeration
B. Sedimentation
Rationale: Sedimentation is the process where suspended solids settle out of
wastewater by gravity, forming sludge.
2. What is the main purpose of aeration in wastewater treatment?
A. Remove pathogens
B. Reduce odor
C. Provide oxygen for microbial activity
D. Remove heavy metals
C. Provide oxygen for microbial activity
Rationale: Aeration supplies oxygen needed for aerobic microorganisms to
break down organic matter.
3. The BOD (Biochemical Oxygen Demand) test measures:
A. Oxygen produced by photosynthesis
, B. Organic matter in water
C. Chemical contaminants
D. Total dissolved solids
B. Organic matter in water
Rationale: BOD indicates the amount of organic matter that bacteria can
biologically degrade, reflecting water pollution levels.
4. What is the primary goal of primary treatment?
A. Remove dissolved chemicals
B. Remove large solids and settleable materials
C. Disinfect wastewater
D. Adjust pH
B. Remove large solids and settleable materials
Rationale: Primary treatment focuses on physically removing settleable
solids through screening and sedimentation.
5. Which microorganism is most important in aerobic wastewater
treatment?
A. Viruses
B. Protozoa
C. Bacteria
D. Algae
C. Bacteria
Rationale: Aerobic bacteria decompose organic matter, forming carbon
dioxide, water, and biomass.
6. Sludge digestion primarily reduces:
A. Pathogens only
B. Odor only
C. Volume and organic content of sludge
D. pH
C. Volume and organic content of sludge
Rationale: Digestion stabilizes sludge by decomposing organic matter,
reducing its volume and potential odors.
, 7. Nitrification in wastewater treatment involves:
A. Conversion of ammonia to nitrite and nitrate
B. Removal of phosphorus
C. Settling of solids
D. Disinfection
A. Conversion of ammonia to nitrite and nitrate
Rationale: Nitrifying bacteria oxidize ammonia first to nitrite and then to
nitrate under aerobic conditions.
8. Denitrification is the process where:
A. Nitrogen gas is converted to ammonia
B. Nitrate is converted to nitrogen gas
C. Nitrite is converted to nitrate
D. Ammonia is absorbed by sludge
B. Nitrate is converted to nitrogen gas
Rationale: Denitrification occurs under anoxic conditions, removing
nitrogen from wastewater.
9. Which chemical is commonly used for disinfection in wastewater?
A. Sodium hydroxide
B. Chlorine
C. Calcium carbonate
D. Sulfuric acid
B. Chlorine
Rationale: Chlorine effectively kills bacteria and pathogens before
wastewater is discharged.
10. The term “effluent” refers to:
A. Raw sewage entering a plant
B. Treated wastewater discharged from a plant
C. Settled sludge
D. Industrial chemicals
B. Treated wastewater discharged from a plant
Rationale: Effluent is the final treated water released into the environment.
Contractor Exam Questions And
Correct Answers (Verified Answers)
Plus Rationale2026 Q&A| Instant
Download Pdf
1. Which process is primarily used to remove suspended solids from
wastewater?
A. Biological treatment
B. Sedimentation
C. Chlorination
D. Aeration
B. Sedimentation
Rationale: Sedimentation is the process where suspended solids settle out of
wastewater by gravity, forming sludge.
2. What is the main purpose of aeration in wastewater treatment?
A. Remove pathogens
B. Reduce odor
C. Provide oxygen for microbial activity
D. Remove heavy metals
C. Provide oxygen for microbial activity
Rationale: Aeration supplies oxygen needed for aerobic microorganisms to
break down organic matter.
3. The BOD (Biochemical Oxygen Demand) test measures:
A. Oxygen produced by photosynthesis
, B. Organic matter in water
C. Chemical contaminants
D. Total dissolved solids
B. Organic matter in water
Rationale: BOD indicates the amount of organic matter that bacteria can
biologically degrade, reflecting water pollution levels.
4. What is the primary goal of primary treatment?
A. Remove dissolved chemicals
B. Remove large solids and settleable materials
C. Disinfect wastewater
D. Adjust pH
B. Remove large solids and settleable materials
Rationale: Primary treatment focuses on physically removing settleable
solids through screening and sedimentation.
5. Which microorganism is most important in aerobic wastewater
treatment?
A. Viruses
B. Protozoa
C. Bacteria
D. Algae
C. Bacteria
Rationale: Aerobic bacteria decompose organic matter, forming carbon
dioxide, water, and biomass.
6. Sludge digestion primarily reduces:
A. Pathogens only
B. Odor only
C. Volume and organic content of sludge
D. pH
C. Volume and organic content of sludge
Rationale: Digestion stabilizes sludge by decomposing organic matter,
reducing its volume and potential odors.
, 7. Nitrification in wastewater treatment involves:
A. Conversion of ammonia to nitrite and nitrate
B. Removal of phosphorus
C. Settling of solids
D. Disinfection
A. Conversion of ammonia to nitrite and nitrate
Rationale: Nitrifying bacteria oxidize ammonia first to nitrite and then to
nitrate under aerobic conditions.
8. Denitrification is the process where:
A. Nitrogen gas is converted to ammonia
B. Nitrate is converted to nitrogen gas
C. Nitrite is converted to nitrate
D. Ammonia is absorbed by sludge
B. Nitrate is converted to nitrogen gas
Rationale: Denitrification occurs under anoxic conditions, removing
nitrogen from wastewater.
9. Which chemical is commonly used for disinfection in wastewater?
A. Sodium hydroxide
B. Chlorine
C. Calcium carbonate
D. Sulfuric acid
B. Chlorine
Rationale: Chlorine effectively kills bacteria and pathogens before
wastewater is discharged.
10. The term “effluent” refers to:
A. Raw sewage entering a plant
B. Treated wastewater discharged from a plant
C. Settled sludge
D. Industrial chemicals
B. Treated wastewater discharged from a plant
Rationale: Effluent is the final treated water released into the environment.
